Effect of citrate on plasma aluminium concentration and aluminium excretion in the rat

Renal Handling of Aluminium

Aluminium (Al) is absorbed from a variety of foodstuffs and medications. Its major route of elimination from the body is in the urine. However, current knowledge concerning its glomerular filtration and, more particularly, its reabsorption/secretion is fragmentary. Most (80-90%) of Al in the plasma is normally bound to protein (mainly transferrin) and is therefore unfilterable; the remainder is bound to low molecular mass compounds, of which citrate appears to be the most important. In vitro determinations using artificial membranes indicate that approximately 10% of Al is filtered at normal plasma concentrations. However, when plasma Al is raised experimentally, its filterability falls, unless the excess Al is complexed with citrate; the aluminium citrate complex appears to be freely filtered. Information on tubular Al reabsorption at normal plasma concentrations is inconsistent. Filtered Al appears to be at least partially reabsorbed, although the reabsorptive mechanisms remain speculative. A consensus is emerging that elevated plasma Al concentrations result in a fall in fractional Al reabsorption, and a recent micropuncture study indicates that under these circumstances the only significant site of Al reabsorption is the loop of Henle.

Renal aluminium handling in the rat: a micropuncture assessment

Uncertainties exist over the glomerular filtration of aluminium and virtually nothing is known about its segmental handling along the nephron. The present study has used micropuncture, combined with electrothermal atomic absorption spectroscopy, to determine directly the aluminium content of glomerular filtrate and of late PCTs (proximal convoluted tubules) and early distal tubules in anaesthetized Munich–Wistar rats infused with three different doses of aluminium citrate (plasma aluminium concentrations, 2.9±0.1, 5.2±0.4 and 10.0±0.9 μg·ml−1 respectively). Aluminium filtration into Bowman's space was found to be considerably greater than that predicted by an in vitro filtration system: in all three groups it was essentially filtered freely. No significant aluminium reabsorption took place along the PCT, but with every dose the FDAl (fractional delivery of aluminium; tubular fluid:plasma aluminium/inulin concentration ratio) was lower at the early distal site than at the late PCT (P<0.001 in each case), indicating net aluminium reabsorption in the loop of Henle. This reabsorption amounted to 19–26% of the filtered aluminium load. In the low- and medium-dose groups, there was no significant difference between FDAl at the early distal site and that in the final urine; however, in the high-dose group, FDAl in the urine (1.02±0.06) exceeded that at the early distal tubule (0.75±0.04; P<0.001), suggesting aluminium secretion in the distal nephron. The results indicate that aluminium loads, when complexed with citrate, are excreted efficiently owing to a combination of glomerular filtration and minimal reabsorption.

Al and Si: their speciation, distribution, and toxicity

OBJECTIVES

In dialysis patients both aluminum (AI) and silicon (Si) may accumulate. Whereas the toxic effects of AI within this population are clearly established, little is known on the role of Si in the development/protection of particular dialysis-related diseases. A clear insight in the protein binding and speciation of trace elements is important to better understand the mechanisms underlying their toxicity/essentiality. Research in this field however is complex and often prone to analytical difficulties and inaccuracies.

DESIGN AND METHODS

In the first part of this review techniques used for speciation studies of AI and Si in biological fluids are discussed. Notwithstanding recent technical advances (a) extraneous metal contamination, (b) unrecognized aspecific binding of metals to proteins, and (c) unwanted interactions with separation equipment such as chromatography columns and ultrafiltration membranes remain important pitfalls and often lead to erroneous conclusions. The factors that determine the speciation of AI and Si and their ultimate tissue distribution and toxicity are dealt with in the second part. Here, experimental data obtained with various speciation techniques are linked to in vivo data on the tissue distribution, localization/toxicity of both elements.

CONCLUSIONS

A model in which the AI tissue distribution/toxicity is mediated by either its citrate or transferrin bound form is proposed.

Gastrointestinal absorption of aluminium and citrate in man

Aluminium (Al) is an abundant terrestrial element, but toxic to tissues, including brain. The body is largely protected because systemic Al absorption is very low and in normal individuals almost all absorbed Al is excreted from the body. However gastrointestinal (Gl) absorption is enhanced by organic acids, including citrate. Aluminium and citrate Gl absorption was measured in three healthy males, aged 40-46. After overnight fast, subjects drank a 100 ml fruit drink containing 280 mg Al and 3.2 g citrate (104 and 167 mM, respectively). Al was measured in timed blood and urine samples by GFAAS and serum citrate by enzymatic assay. Blood Al peaked by an increase of 13 +/- 2.1 micrograms/l after 87 +/- 19 min then fell slowly over 24 h. Plasma citrate peaked after 32 min, returning to baseline by 90 min. Al was excreted at a constant rate for the first 24 h, 0.4% of the dose being excreted in urine by this time. It is unlikely that Al is absorbed as Al citrate because the blood citrate peak preceded the Al peak by 45-60 min.

Renal excretion of aluminium in the rat: effect of citrate infusion

When aluminium is administered intravenously to rats, the speciation of the aluminium has a major effect on its renal excretion. Aluminium administered as citrate is much more effectively excreted than that administered as chloride or sulphate. This suggests that citrate could be therapeutically useful in patients who have been exposed to aluminium. Accordingly, we have performed two series of experiments in rats, in which a citrate infusion (intravenous), was begun either immediately after, or one hour after, the administration of an intravenous aluminium sulphate bolus. Both protocols led to markedly enhanced aluminium excretion compared to controls in which only 0.7% NaCl was infused. The enhancement of aluminium excretion was 783% if citrate infusion was begun immediately after aluminium administration, and 335% if the citrate infusion began after an hour delay. The increased excretion was due to an increase in the freely filterable fraction of aluminium. In the control experiments, in which aluminium sulphate administration was followed by 0.7% NaCl infusion, aluminium was found to be deposited in the liver. Administration of citrate one hour after the aluminium bolus did not reduce this liver deposition. The results indicate that a fraction of the plasma aluminium is accessible to the citrate infused and can thereby be converted into a filterable form which can be excreted. It appears that, for maximum therapeutic effect, citrate should be infused as rapidly as possible after an aluminium load, to limit aluminium binding to ligands which allow it to enter cells.

Aluminium deposition in liver and kidney following acute intravenous administration of aluminium chloride or citrate in conscious rats

1. Plasma, urinary, liver and kidney cell aluminium (Al) levels were monitored in the rat, 1h after intravenous administration of 29630 nmol (800 micrograms) Al as either Al chloride or as Al citrate (Al chloride plus excess sodium citrate). Al levels were measured in plasma, urine and liver by atomic absorption spectroscopy (AAS). Liver and kidney Al content was measured at the cellular and subcellular level by electron probe X-ray microanalysis (EPXMA). 2. Urinary excretion of Al was significantly higher (P < 0.01), when Al was given as the citrate than as the chloride. After 1h, plasma Al levels were significantly lower in the Al citrate group than the Al chloride group (59 +/- 3.7 vs 877 +/- 214 nmol ml-1, respectively; P < 0.01). 3. Al concentrations were significantly higher in the livers of rats receiving Al chloride (818 +/- 252 nmol g-1 wet weight; P < 0.05), than in either control or Al citrate groups (122 +/- 41 and 107 +/- 26 nmol g-1 wet weight, respectively). Al concentrations derived from EPXMA measurements were in agreement with AAS values for the three groups, with significantly higher Al concentrations in the Al chloride group (1.7 +/- 0.4 nmol mg-1 dry weight; P < 0.05) than in the control or Al citrate groups, where Al was not detectable. EPXMA analysis showed that Al was distributed in all liver organelles analysed (cytoplasm, mitochondria, nucleus, ER) and was not preferentially taken up by any one organelle in Al chloride treated rats. 4. Significant amounts of Al were found in cytoplasm and mitochondria of proximal tubule cells of rats given Al citrate (0.64 +/- 0.15 and 0.80 +/- 0.11 nmol mg-1 dry weight, respectively), but not in nuclei or lysosomes of these cells. Al levels were not detectable in control kidneys, in proximal tubule cells after Al chloride administration or distal tubule cells after either Al treatment.

Renal filtration and excretion of aluminium in the rat: dose-response relationships and effects of aluminium speciation

The known toxicity of aluminium, and the toxicity of agents (such as desferrioxamine) used to remove aluminium from the body, has prompted us to investigate whether there may be ways of enhancing aluminium excretion by exploiting the normal renal handling of aluminium. Aluminium (as sulphate or citrate) was administered intravenously to conscious rats at doses ranging from 25 micrograms (0.93 mumol) to 800 micrograms (29.6 mumol) aluminium, and aluminium excretion was monitored over the following 2 h. Measurements of the filterability of aluminium from the rat plasma, and the glomerular filtration rate (inulin clearance), enabled us to calculate the filtered load of aluminium, and hence determine aluminium reabsorption. At all doses of administered aluminium, that administered as sulphate was excreted less effectively than that administered as citrate. This difference was attributable to the much greater filterability of aluminium administered as citrate. However, for any given filtered load, the excretion of aluminium administered as citrate was not significantly different (in either fractional or absolute terms) from the excretion of aluminium administered as sulphate. It seems likely that, following aluminium sulphate administration, the filtered aluminium may be an aluminium citrate form which is then reabsorbed in the same way as aluminium administered as citrate. It is thus apparent that aluminium removal from the body could be further enhanced if it were possible to prevent the tubular reabsorption of the aluminium species which is so effectively filtered following aluminium citrate administration.

Aluminium uptake from some foods by guinea pigs and the characterization of aluminium in in vivo intestinal digesta by SEC-ICP-MS

The uptake of ingested aluminium (Al) from food items commonly consumed in a normal human diet was investigated by feeding five test diets to guinea pigs. Al concentrations were measured in the femur, brain, kidney and upper intestinal contents. Consumption of these diets did not lead to elevated Al levels in brain. Levels of Al in the bone were elevated in animals fed sponge cake with a permitted Al-containing additive, and the presence of citrate as orange juice enhanced bone deposition and increased kidney Al levels. Less than 1% of Al in the upper intestinal contents was found in the soluble fraction, and characterization by SEC-ICP-MS indicated that this Al was not present as Al-citrate.